National Energy Technology Laboratory
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Deep Drilling Improvements
 

As a drillstring reaches deeper and deeper into the earth, two things generally happen: (1) more challenging drilling problems occur more frequently and (2), dealing with them takes more and more time. "Time is money" in any drilling operation, but this is particularly true when the specialized equipment required for deep drilling is a part of the daily cost of operation. The result is that in very deep wells, drilling the last 10 percent of the hole can consume as much as 50 percent of the total drilling cost.

Why does depth mean more problems? In the subsurface, four things increase with depth: temperature, formation fluid pressure, rock hardness, and of course, the distance between the bit doing the work and the people controlling the system. As that distance increases so does the difficulty in maintaining effective communication between the surface and the bottom of the hole. Temperatures that can reach above 350 degrees Fahrenheit, and pressures that can exceed 10,000 psi seriously challenge the capabilities of the sophisticated downhole equipment used in modern drilling, particularly electronic components and motor seals. To
make matters worse, this high temperature/high pressure environment accelerates the impact of any corrosive fluids that may be encountered at greater depths (e.g., saturated brines and acid gases like carbon dioxide and hydrogen sulfide).

 

Smart Drilling Systems allow drillers knowledge of what is happening at the bottom of the hole while drilling  

 

Rock hardness and abrasiveness typically increase with depth, leading to a lower rate of penetration (ROP) and shorter bit life. Penetration rate can be reduced to only a few feet per hour at an operating cost of tens of thousands of dollars per day for a land rig, or hundreds of thousands of dollars per day offshore. Drill bits that cost $50,000 to $100,000 each may last only days or hours, and tripping the drillstring out of and into the hole each time the bit needs to be replaced or tools need repaired leads to hours or days of nonproductive time. Many drilling problems can arise during these trips, compounding their impact and lengthening the delays.

Also, as the depth of the hole increases, control over well bore trajectory and the placement of casing become increasingly difficult, as does the efficient removal of drill cuttings. The difficulty of cementing wells increases with depth, not only due to the increased potential for poor mud displacement and lost circulation, but also due to the effects of higher temperature on cement setting behavior.

The cost of drilling and completing deep wells rises with the increase in the number and frequency of drilling problems and the increasing length of time it takes to correct them. But costs also are greater simply because of the physical requirements of a deep well. Deeper holes require longer drillstrings and longer casing strings and thus require rigs rated to support such weights. These rigs are large and expensive, and only a limited number are available, increasing their cost.

Deep Drilling Research and Development Needs
For the nation to benefit from deep gas reserves, we need to be able to economically drill and complete deep wells. Reducing the cost of deep drilling will require that we reduce the amount of time it takes to safely drill a well, by reducing the number of problems drillers face and improving their ability to deal with them quickly. To do this, we need to reduce the need for trips, increase the life of bits and other drilling assembly components, improve the reliability of drillstring tubulars under extreme conditions of temperature and pressure and, perhaps most importantly, increase the driller's knowledge of what is happening at the bottom of the hole, when it is happening. This last capability is dependent on the development of tools that can sense conditions at the bottom of the hole and instantly transfer that information to the driller in "real time," allowing the driller to react quickly or in some cases, anticipate problems and avoid them.

Over the last decade the drilling industry has advanced its ability to drill shallow wells more efficiently. But if wells/rig or footage/rig can be used as a rough metric for efficiency, the industry's ability to drill deep wells has not improved, and in the case of gas wells, performance has dropped.

To address these issues, the DOE kicked off the Deep Trek Program in March 2001 with a workshop to capture industry's perspective on specific R&D needs for improving our ability to economically drill and complete deep gas wells, and to determine how the DOE can best collaborate with industry to meet these needs. Hosted by the National Energy Technology Laboratory (NETL) and Sandia National Laboratories, the workshop focused on specific technology areas: advanced “smart” drilling systems, drilling diagnostics and sensor systems, drilling and completion fluids, and completion-based well design. During the workshop the following R&D tasks were identified as the top three priorities in each of these areas:

Advanced Smart Drilling Systems

  • Improve the accuracy, reliability and cost-effectiveness of “real time” data acquisition systems operating under extreme conditions

  • Develop higher data transfer rate telemetry systems for transferring data between the bottom of the hole and the surface

  • Develop a rig operator decision support system with open architecture that incorporates surface and downhole data from improved data sensors with algorithms that help drillers anticipate problems

Drilling Diagnostics and Sensor Systems

  • Improve the accuracy, reliability and cost-effectiveness of systems for acquiring and validating downhole diagnostic parameters (weight on bit, torque at bit, borehole assembly condition, downhole motor parameters

  • Develop electronic assemblies and power supplies capable of high performance under high temperature

  • Develop a real time pore pressure detection system utilizing an improved seismic-while-drilling tool and incorporating automatic kick detection and control

Drilling and Completion Fluids

  • Develop economic equipment and test procedures to better characterize the contribution of drilling fluid to well-bore stability, as well as models that can accurately capture the mechanical/chemical interactions of the fluid with the formation

  • Develop environmentally acceptable methods for disposal of drilling and completion fluids and demonstrate them for regulatory acceptance

  • Develop water-based drilling fluids that can serve as alternatives to oil-based fluids

Completion-Based Well Design

  • Develop and apply high-temperature/high-pressure sensors and information tools to the drilling and completion process

  • Accelerate development of temperature-resistant composite tubulars, expandable tubular systems and tubulars with embedded sensors/data conduits

  • Develop temperature-resistant downhole solutions to surface production problems (e.g., downhole separators and injection systems, “smart well” systems for monitoring and control)
     

Table of Problems

Problem High Temperature High Pressure Rock Properties Physical Length of Drillstring
Low ROP     x  
Short bit life x   x  
Time required for trips       x
Downhole motor failures or limits x x    
MWD/LWD tool failures of limits x x    
Casing wear     x x
Drilling fluid degradation x      
Hole cleaning difficulties       x
Vibration, tubular limits       x